The interaction between an amplifier and a speaker can be quite complex. While speaker sensitivity is the most critical factor, parameters such as speaker impedance and phase can also be critical. The JBL's you're considering, however, have very well-behaved impedance and are easy to drive. If the speaker sensitivity is rated at 90dB/1W/1m, and you'd like to achieve a volume level of 110dB (quite loud) you'll need at least 100WPC. The easy calculation is: